Comparison of lead adsorption characteristics onto soil-derived particulate organic matter versus humic acid

Differences in the composition and properties of soil organic matter (SOM) components with different decomposition stages determine their ability and mechanism to sequester heavy metals in contaminated soils. However, very little research has emphasized the adsorption properties of heavy metals on humic acid (HA) versus particulate organic matter (POM). X-ray absorption fine structure (XAFS) spectral analysis, in combination with Fourier transform infrared (FTIR) spectroscopy and energy dispersive X-ray spectroscopy (SEM-EDS), was applied to investigate the structures and compositions of tropical secondary soil-derived POM and HA and their adsorption characteristics for lead (Pb). Results revealed that HA was characterized by high aromaticity and cation exchange capacity (CEC) and a smoother structure, while POM mainly consisted of aliphatic components with comparable polarities with high H/C and (N+O)/C ratios was embedded with soil minerals (e.g., gibbsite, silicate, and phosphate) and possessed a larger specific surface area and high content of Si-O-Si and free -OH groups. The equilibrium adsorption data for HA followed fit the Langmuir model with a maximum capacity of 72.57 mg g−1 while those for POM the Freundlich model with a maximum capacity of 22.27 mg g−1. Freundlich-n and KF values suggested that sorption of Pb on POM displayed a nonlinear isotherm and lower initial affinity relative to that on HA. XAFS results showed that two major Pb species similar to Pb(C2H3O2)2 and PbO were formed in Pb-loaded HA, while four major Pb species similar to Pb-loaded Al2O3, Pb(C2H3O2)2, Pb3(PO4)2, and PbSiO3 were formed in Pb-loaded POM. Based on desorption and spectroscopic studies, Pb sorption sites on HA were primarily ascribed to metal exchange and surface inner-sphere complexation with phenolic hydroxyl and carboxyl functional groups, while Pb sorption on POM involved coprecipitation, electrostatic cation exchange, and inner-sphere complexation with complexed organic functional groups and mineral oxides, and these processes accounted for 65% and 48% of the total sorbed Pb on HA and POM, respectively, and led to a series of Ca2+, Mg2+, K+, and Na+ releases during the sorption process. In addition, the contribution of hydrogen bonding to Pb sorption on HA and POM was significant, accounting for 34% and 46%, respectively. Thus, differences in sorption mechanisms between HA and POM help us choose effective strategies for enhancing Pb(II) removal and environmental risk assessment in aqueous (e.g., wastewater) and soil environments.